1) Cloned Moloney murine sarcoma virus (MSV) proviral DNA, containing the acquired cell sequence v-mos and the long terminal repeat (LTR) provirus transcription control element, transforms cells efficiently in DNA transfection assays. In contrast, a normal mouse cell DNA sequence (c-mos) homologous to the transforming sequence of MSV was shown to be inactive in this assay. The transforming potential of c-mos was activated by forming recombinant molecules containing the LTR positioned from 500-1500 base-pairs upstream from the 5' end of the cellular mos. These results suggest a model for oncogenesis whereby a normal cell gene with transforming potential is activated by a transcription control element to express its transforming phenotype. A normal human cell homolog of mos was cloned and compared to the MSV and mouse v-mos sequence. Heteroduplex analyses revealed that under non-stringent conditions the human mos was co-linear with the mouse mos. However, most of the six base restriction enzyme recognition sites of v-mos are not conserved in the human homolog.